On the role of bimolecular reactions in chemical activation systems

Abstract

The kinetics of chemically activated intermediates in complex-forming bimolecular reactions can often be described in terms of the branching ratio between their unimolecular decomposition channels and the collisional stabilization under steady-state conditions (D/S). This requires that the stabilized part of the population does not contribute to the decomposition rate, which is true only for reaction times shorter than the thermal lifetime of the intermediate. As these intermediates, however, are often radicals or other highly reactive species, they can undergo consecutive bimolecular reactions. Here two limiting cases are conceivable: (i) If the bimolecular steps are slow, mainly stabilized intermediates react, and their population is prevented from being built up, i.e. their thermal decomposition is suppressed. In this case the steady-state description in terms of D/S remains adequate also for reaction times longer than the thermal lifetime of the intermediate. (ii) If the bimolecular reactions are extremely fast, even highly excited intermediates react and are removed from the population. Consequently, the observed relative yield of the unimolecular channels, F, drops below the steady-state value. The branching between decomposition and stabilization of a chemically activated intermediate, therefore, can depend on the rate of its bimolecular consecutive reactions, a fact which is often overlooked in the kinetic analysis of such systems. In the present work we investigate the dependence of F on the rate of these bimolecular steps in a general way by means of a master equation. A simple method is presented to estimate the parameter range, where the steady-state approach in terms of D/S remains adequate if bimolecular reactions of the intermediate occur. The problem is exemplarily treated for the reaction between chemically activated but-2-yl radicals and hydrogen atoms.

Consecutive Chemical Activation Steps in the OH‐Initiated Atmospheric Degradation of Isoprene: An Analysis with Coupled Master Equations

“…Here, the symbol * indicates that the intermediate C is chemically activated, i.e., obeys a nonthermal energetic distribution, which is influenced by reactions (a)–(c) and collisions with the bath gas. For general aspects of chemical activation, the reader is referred to the monographs and for the influence of consecutive bimolecular steps to .…”

“…is most conveniently obtained by directly solving R a F = JN ss for N ss (routines banbks and bandec from ) and subsequent normalization. For more technical details of our approach, see .…”

“…For general aspects of chemical activation, the reader is referred to the monographs [11][12][13] and for the influence of consecutive bimolecular steps to [17,[27][28][29][30].…”

Kinetics of the unimolecular reaction of CH2OO and the bimolecular reactions with the water monomer, acetaldehyde and acetone under atmospheric conditions

“…Our general approach is described in detail elsewhere, [43][44][45] and only a brief outline is given here. Our general approach is described in detail elsewhere, [43][44][45] and only a brief outline is given here.…”

Global uncertainty analysis for RRKM/master equation based kinetic predictions: A case study of ethanol decomposition

“…The authors predicted a greater uncertainty in the rate constants for R2 and R3 than for R1, given the difficulty of measurements for the low branching channel and the strong sensitivity of the corresponding theoretical predictions to the details of the energy transfer process (depending on both the energy transfer parameters and the energy gap between the barriers of competing channels). Very recently, Kiecherer et al [14] studied ethanol pyrolysis with shock-tube/time-of-flight mass spectrometry experiments and computed the rate coefficients by the master equation method with a stepladder energy transfer model [40].…”